The Clean Air Act of 1970 and the Noise Control Act of 1972, both as amended, together with the dramatically increased fuel costs of recent years, have combined to create the need for continually improving exhaust systems for heavy-duty trucks. The need for cleaner emissions is often at odds with the need for more silencing. The truck smoke and gaseous emission regulations, plus truck horsepower and fuel economy needs, have resulted in the use of diesel turbo-charged engines with significantly increased engine air demands, but no increase has been permitted in either intake restriction or exhaust system back pressure. One way of seeking to improve exhaust noise level is to utilize silencing features which increase back pressure but this option is not always desirable.
For 1982, manufacturers of heavy-duty diesel trucks will be required to limit truck noise to a sound level not to exceed 80 dBA at 50 feet. This is a drive-by test and all noise generated by the truck, including exhaust noise, is included in the measurement. Because exhaust noise is generally more easily controlled than engine mechanical or driveline noise, truck manufacturers will require mufflers which will silence exhaust noise down to the 65-68 dBA range, and at the same time will desire substantially less flow losses than current systems. This is about a 15 dBA improvement over 1973, a 10 dBA improvement over 1975 and a 5 dBA improvement over 1978.
The state of the art at the present time is represented by the structures shown in the Rowley et al. U.S. Pat. No. 3,672,464, issued June 27, 1972; the Hunt U.S. Pat. No. 4,064,962, issued Dec. 27, 1977; the Nordlie U.S. Pat. No. 4,143,739, issued Mar. 13, 1979; and variations of the three devices. The Rowley patent, which is assigned to the same company as the present invention, and which was released to production in 1970, utilizes a sonic choke and straight-through flow. Further, a flow path around the sonic choke is provided to lower the back pressure. In about 1971, a model was utilized in which all of the flow went through the sonic choke, which then utilized a larger diameter throat in the venturi. Some additional silencing was also achieved in later years by increasing the diameter of the muffler and, in some styles, increasing the length of the body. These sonic choke-type mufflers with straight-through flow met the silencing needs of the time and were very fuel efficient because of their low back pressure.
In about 1974, it became necessary once again to improve the silencing. The Rowley type muffler was again modified, this time by fully capping the inlet tube with a baffle so that all exhaust flow was diverted through a muffler chamber before entering the sonic choke. This substantially increased the back pressure, but it did achieve the necessary improvement in silencing. That muffler, with the capped inlet tube and the single sonic choke, still works very well on four-cycle engines. It will silence current four-cycle diesel engines to the 65 dBA level, but the back pressure is somewhat high.
A bigger problem, however, is the silencing of two-cycle diesel engines. The two-cycle engine operates in a different frequency spectrum. In addition, the two-cycle engine requires a positive displacement blower and utilizes an exhaust turbo charger. Thus, while certain state of the art mufflers can be used to silence four-cycle engines, mufflers of conventional size are normally not capable of silencing two-cycle engines to the necessary dBA level for 1982. Our limited tests appear to show that the Hunt-type mufflers can silence into the low 70's and that the Nordlie-type mufflers can silence into the high 60's on four-cycle engines and into the high 70's on two-cycle engines. The modified Rowley-type mufflers, utilizing a single sonic choke, can silence four-cycle engines into the mid 60's and two-cycle engines into the low 70's. Thus, utilizing present state of the art mufflers, the four-cycle engines can be silenced to the necessary level but at the cost of an undesirably high back pressure, but the two-cycle engines cannot be effectively silenced to the 65-68 dBA level.
To meet today's noise level requirements, truck mufflers are typically nine to ten inches in diameter and about 45 inches long. It was thought, prior to the present invention, that in order to provide the silencing required for 1982, it would be necessary to both make technical improvements and increase the muffler size, i.e. to increase the diameter to 12 inches or else increase the length to about 60 inches, while at the same time double wrapping the muffler to control shell noise and adding other noise reduction features such as packing. Even if these prospective changes to the existing structures had sufficiently reduced the noise level, however, they would have undesirably increased the size of the mufflers, made them more expensive to manufacture, and would probably have increased the back pressure.
In seeking to design a muffler which would adequately silence the two-cycle diesel engines for 1982, I tried many styles of mufflers and in particular, several which utilized a sonic choke. One variation employed the capped inlet tube, with the outlet offset to provide a double-S flow pattern. In other variations, specific chambers were tuned and the capped inlet tube was used with and without a sonic choke. None of these variations showed enough promise. Even those combinations which substantially increased the back pressure did not result in any substantial reduction in noise level. It was only after I made a substantial departure from the prior art that the necessary reduction in noise level was achieved. I discovered that a muffler having two sonic chokes in series, with the two sonic chokes arranged in combination with three chambers, resulted in substantial improvement in performance. In this design, all of the exhaust flows through the tube assembly carrying the sonic chokes and none of the flow is bypassed through the chambers, which helps control shell noise. The basic design, which can be varied by adding additional chambers, or by adding absorptive packing, provides a muffler which will effectively silence the two-cycle diesel engine down to the 65-68 dBA level, while at the same time keeping back pressure at an acceptable level. Tests have shown that the improved silencing can be achieved at about half the allowed back pressure. This reflects a substantial reduction in muffler pressure drop because about one-third of the allowable back pressure is generally the result of exhaust system piping loss. As compared to state of the art muffler designs as applied to two-cycle diesel truck engines, the present invention has provided almost a 10 dBA further reduction in noise level, which corresponds to a 90% reduction in sound energy, without increasing the size of the muffler. Mufflers according to the present invention can thus be used by truck manufacturers on both two-cycle and four-cycle engines to meet the 1982 noise requirements. Because of its simplicity, and because it can be so easily combined with other muffler components, the present invention may also provide a basis for further reductions in vehicle noise levels.